Variable fuel cell power system for generating electrical power

ABSTRACT

The present invention provides an apparatus and methods for variably supplying power from a stand-alone fuel cell power supply system including a power conversion unit, a power switching unit and a load control unit. Preferably, a controller manages system configuration to switch the loads, the power conversion and the delivery between the two without reducing capacity by redundantly backing up each individual portion with a bank of at least two modules for each unit. Preferably, controller actuated devices are triggered automatically in response to monitors that sense performance operating parameters and detect values operating outside a threshold range. Preferably, each of the units and the components in each unit are banked in a plurality of modular units so that individual converters may be interchanged, individual fuel cells may be interchanged, individual controllers may be interchanged, and individual storage units such as batteries may be interchanged to ensure proper operation of each bank despite changes or failures in individual components of the bank. The power system may include couplers for connecting the system to dedicated outlets such as an NEC standard 120/240 volt building circuitry. The invention monitors multiple elements from fuel supply, fuel cells, converters, storage units, and controls as well as the loads for balanced, reliably robust, high power quality, and independent power supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/247,067filed Sep. 19, 2002, which is now U.S. Pat. No. 7,261,962, which is adivisional of U.S. application Ser. No. 09/541,996, filed Apr. 3, 2000,which is now U.S. Pat. No. 6,503,649. The entire disclosure of the priorapplication is considered as being part of the disclosure of thisapplication and is hereby incorporated by reference.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to electrical power supply from a variablefuel cell power supply system for reliably generating ac and dc power asa stand alone unit.

2. Background Art

The availability, defined as percentage of run time over total time; thereliability, determined as the number of occurrences of failure per unittime of operation; and the durability, defined as service life, ofpreviously known fuel cell generators are not sufficient for afree-standing, continuously operating, dynamic load-following, localfuel cell application using currently available manufactured componentsor even improved components.

Previous claims to fuel cell availability, reliability and durability,as reported in newspapers, magazines, and technical journals, arespecific to a particular existing or envisioned application. Forinstance, the durability needs of a fuel cell automobile engine (e.g.,5,000-10,000 hrs) with its intermittent operation may not be acceptablein a residential application with continuous operation (e.g.,45,000-90,000 hrs) requirements. In addition, a fully acceptable 85-90%operating availability for a manned, grid backed, utility site would notbe accepted by the general public for a local, unmanned, residentialunit (where expectations are more likely to be one loss every 5-10years). A NASA space fuel cell application using pure stored liquidhydrogen and oxygen cannot be relied upon by skilled artisans toestablish how to improve reliability to required levels for afree-standing, re-formed fuel, ambient-air terrestrial application,particularly under dynamic and transient operation.

Improvement efforts focus on components rather than systems to solveavailability, reliability, durability, and excessive cost problems. Anon-exhaustive list of specific Proton Exchange Membrane (PEM) fuel cellproblems currently under attack include: carbon monoxide/dioxidecatalyst poisoning from reformed hydrocarbon fuels, membrane hydrationcontrol for dimensional stability and ionic conductivity, membranephysical integrity for pressure differential and expansion orcontraction stress failures, membrane dehydration and electronicfailures caused by hot spots from poor heat dissipation, and fuel cellcontamination caused by incomplete fuel reforming and/or cleanup. Mostfuel cell problems are accentuated by operational dynamics and/ortransients. Conventional wisdom demands that systems must shutdownprocesses in the event of hydrogen leakage. In addition, manufacturingprocess variability for membranes, electrodes, and Membrane ElectrodeAssemblies (MEA's) result in a lack of consistency or repeatability inperformance (or operation) for assembled fuel cell stacks.

In an attempt to solve these problems, previous research and developmentefforts have focused on components. New materials for membranes, bipolarplates, catalysts, composites, and electrodes have been developed.Geometry changes, including stacked plates, round, hexagonal, and othershave been tried with varying success. Improvements by studying thechemistry of contamination, chemical process stability, chemicaldegradation, chemical depletion, and alternative chemistries, have beensought. New processes for manufacturing components have focused on costreduction, tolerances, and uniformity. Improvements in operationalsafety and the effectiveness of system monitoring and control have beensought through the development of controllers and sensors that possessintegrity during variations in pressure, temperature, andhumidification. The need to use fossil fuels, because of the lack of ahydrogen distribution infrastructure, has caused development of storage,distribution, reformer, multi-fuel reformer, and contaminant clean-upequipment. Finally, improvements in ancillary fuel cell generatorcomponents such as energy storage units, electrical converters,compressors, pumps, and manifolds have been tried with varying success.

There are also conflicting issues that require different trade-offs fordifferent applications. Previously known improvements, such as thinningmembranes to increase conductivity and fuel cell power density, reducesreliability by compromising physical integrity. Membranes expand andcontract as much as 20% between their dehydrated and fully hydratedstates, creating opportunities for physical failure due to stress,strain, creep and tear. In addition, gas pressure differentials acrossthe membrane are more likely to cause membrane punctures. When reducingmembrane dimension, membrane thickness and uniformity becomes moredifficult to control. Lightened catalyst loading decreases cost andreduces durability due to increased susceptibility to catalyst poisoningfrom fuel or oxidant impurities.

In addition to the fuel cell stack issues, the remaining plant devicesalso contribute to the operational, reliability, and durabilityproblems. For example, inverter failures have significantly reduced theavailability and durability of prototype fuel cell generators andcommercial Uninterruptible Power Supply (UPS) installations. Both singlefuel and multi-fuel reformers, using commonly available fuels, are stillin the R&D stage for attaining hydrogen-rich and contaminate-freeoperation, particularly under transient conditions. Batteries have wellknown duty cycle and durability issues that have led researchers in thefield toward storage alternatives such as ultra capacitors and flywheels.

Virtually all major automotive manufacturers are targeting 2004 for theintroduction of commercial Proton Exchange Membrane fuel cell engines ofa size that outputs approximately 50 kW. Fuel cells becomecost-competitive with the internal combustion engine at approximately$35-$50 per kW. A large scale phosphoric acid fuel cell (PAFC)stationary commercial supply system is produced by ONSI, the PC25; andBallard Power Systems has demonstrated a 250 kW PEM prototype. Thetarget market for these products are utilities, industries, hospitals,and commercial establishments because of their size. A small scalestationary/residential system has not yet been commercialized, althoughPlug Power has set a residential PEM fuel cell commercialization targetof 2001 for a supply output size of approximately 7 kW, while APC,Avista, Energy Partners, H-Power, and NPS have all stated intentions toenter this market. Fuel cell generators become cost-competitive atapproximately $500-$900 per kW in this market.

Fuel cell developers are looking for synergy between thetransportation/automotive and stationary product markets. However, theprevious developments do not establish that improvements and costreductions will always be directly transferable betweenproducts/markets. Auto manufacturers, suppliers and their strategicpartners are addressing problems through research and development at thecomponent level and through the simplification of systems by componentreduction, however, transportation application requirements allowtradeoffs that are detrimental to stationary applications.

Large stationary fuel cell system developers are accepting the lowreliability specifications of existing utility generating equipment. Theexception is a developer (SurePower) who is targeting high-value powerquality with the application of ONSI Corporation's 200 kW PC25 units.However, the SurePower approach relies upon rotary equipment as well asgrid connection and generators to compensate for fuel cell productioninconsistencies during dynamic and transient load changes. Some smallStationary/Residential system developers including some fuel cellsuppliers and their strategic partners appear to be addressingreliability and durability problems with a grid interface for alternatesupply where the grid becomes an energy storage buffer and backup forthe fuel cell.

Fuel cell technology is generally touted for reliability because thereare no moving parts and availability exceeds that experienced for themost reliable utility generators. However, the current state-of-the-artfuel cell generator falls short of the needs for stationaryfree-standing and/or high power quality markets. The ancillary equipmentneeded to support the fuel cell in an environment with no hydrogen fueldistribution infrastructure, and a need to convert the dc output to acfor most applications, adds complexity and additional failure modes thatreduce availability significantly. Most components are not market mature(reformers, sensors, controls, membranes, etc.) and their reliability isunproven. Where ancillary equipment is mature (inverters, batteries) thedevices are not known for their reliability and durability and do notprovide for secure, unattended operation.

Much of the current research and development effort is targeted atimproved fuel cell materials, catalysts, and fuel cleanup all driven byreduced cost targets. However, many of the cost reduction efforts havepotentially degrading effects on the fuel cell reliability,availability, and durability experienced to date. The use of thinnerfuel cell membranes reduces cost and weight while sacrificing physicalstrength and integrity. Lower catalytic loadings reduce costs whileincreasing the risk of catalyst poisoning and perhaps reducingdurability. New materials in bi-polar plates reduce costs but maydegrade operating characteristics and conductivity. New materials andthe processes producing them could introduce impurities that woulddetrimentally affect the long term stability of the fuel cell's chemicalreactions. Even the strongest supporters of fuel cells acknowledge theperformance risks associated with taking this technology to market at acompetitive price.

In a commercial utility grid, the interconnected grid is divided intocontrol areas and each area is the assigned responsibility of a specificutility. The control strategy uses the mechanical inertia of rotatingelectrical equipment, generator governors set at 60 cycles, andsupplemental generator control (using unloaded spinning reserve), tomaintain the area control error (ACE) and grid frequency withinspecified ranges under normal dynamic loading conditions. In simplifiedform, ACE equals the difference between the area's load and the sum ofthe area's generation plus purchase power contracts minus its salescontracts. A non-zero ACE indicates an unbalance in the load and supply.For supply deficiency, a signal derived from ACE is used to loadspinning reserve, and for supply excess, the ACE signal is used tounload generation. The inertia of rotating equipment is used to bufferthe system with rotational frequency rising above 60 cycles when supplyexceeds demand and frequency declining below 60 cycles when demandexceeds supply—thereby achieving a new energy balance between supply andload at the new frequency. An uncontrolled unbalance in grid supply anddemand (an unstable condition) would cause widespread physical damagewhen rotating equipment is forced to operate outside the rotationalspeed design parameters (typically 57-62 cycles). These controls are notreadily applicable to a fuel cell stand alone power supply.

SUMMARY OF THE INVENTION

In light of a moving and hidden, often proprietary, reference from whichto establish the reliability, availability, and durability of anintegrated commercial product, the present invention overcomes theabove-mentioned disadvantages by providing a variable system design toallow the “tailoring” of the product to attain the appropriate balanceof reliability, availability, durability, operational stability, andcost that will create a robust, stand-alone, small-scale product capableof handling transient conditions and particularly adapted to satisfy theresidential consumer without rotary equipment buffering from an outsidepower source.

As used in the disclosure, a free-standing local generation system meansa variable fuel cell power supply system that simulates the powerreliability of a power grid, but generated within the physicalboundaries of a service zone such as a local residence, building, orplant. The stand alone system of the present invention includesredundancy that replaces the multi-feed network of the commercial powergrid, to provide a robust power source that maintains the requiredenergy balance within this new, smaller, network as transient conditionsoccur. The present invention provides a free-standing commercialnon-rotating generation system, with no mechanical inertia, that meetsthe requirements of the residential customer for availability,reliability and durability because adaptive controls, switchableredundant capacities, and switchable loads mitigate the problems causedby imbalances and partial system failures.

Commercial free-standing generators using rotating prime mover systems,have, at best, 80-90% availability, and are intended for grid backup orintermittent use. In all well-designed equipment according toconventional wisdom, excessive or prolonged energy unbalances are eitherabsorbed by controlled heat dissipation or are protected against byover-temperature, under-temperature, over-speed, under-speed, overload,or short circuit detections and the conventional response is to forceequipment shutdown for safety and damage protection. Completeinterruption of service is not necessary to mitigate problems outsidethe range of the invention's adaptive controls because individual loadsor load combinations can be switched off or thrown over to return thesystem to a state where the adaptive control regains effectiveness andonly a partial interruption occurs, thereby improving availability.Furthermore, the adaptive control system will allow the customer todetermine load priorities so that the least essential loads areconsidered first for interruption. Thus, the present inventioncompensates for variability, with adaptive control of system functionsincluding the activation or deactivation of power supply modules and theswitching or throw-over of load circuits.

In addition, although inverters have been used in uninterruptible powersupply (UPS), electronic clean power, alternative energy, and renewableenergy systems, the essence of these systems is to couple storageequipment and non-continuous supply sources to the grid in such a way asto produce a more continuous supply. For example, the PK Electronicsapplication is a UPS targeted at local/personal computer systems andlocal computer network loads smaller than 16 kVA at 120 vac. The SurePower System is a high-availability system implemented in “switchgear”modules using the ONSI Corporation 200 kW PC25 phosphoric acid fuel celland rotary equipment for mechanical inertia (flywheels). Suchimprovements have not dealt with the interface of fuel cell technologiesand their problems in a free-standing residential generation system.Reliability problems with inverters as well as other components havespawned designs needed by computer and other power sensitive processesthat are not readily adapted to a freestanding, variable fuel cell powergeneration system.

The controller preferably includes logic in this adaptive control thatmay be used to extend the “apparent” life of the components by managingtheir activation and deactivation so that load-life burdens for similarcomponents or cells may be equalized. Furthermore, load-life burdens fordissimilar service rating components or fuel cell chemistries can beapportioned to extend the overall life expectancy of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood by reference tothe following detailed description of a preferred embodiment when readin conjunction with the accompanying drawings in which like referencecharacters refer to like parts throughout the views and in which:

FIG. 1 is a schematic view of a fuel cell power supply systemconstructed according to the present invention;

FIG. 2 is a schematic view similar to FIG. 1 but showing a modifiedlayout in accordance with the present invention;

FIG. 3 is a schematic view similar to FIGS. 1 and 2 but showing amodified layout in accordance with the present invention; and

FIG. 4 is a graphic representation of power surge compensation andenergy balance provided on a stand-alone system according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention provides a variable fuel cell power system (VFCPS)10 in several embodiments for an adaptive design of a stand alone supplysystem that reconfigures itself into a viable supply system undernumerous contingencies so that points of failure do not incapacitatesupply output. The system selectively adjusts to a customer's prioritiesfor service continuity when a reduction of supply capacity isexperienced or is selected by adaptive adjustments. The flexibility ofthe design allows selection of the degree of redundancy, selection ofthe overall capacity, resizing for increased usage, and selection of thereliability or availability desired. The ability to retrofit and resizein the field protect the purchaser's investment and increases thelikelihood of early commercial adoption of the technology. Themodularity and scalability is applied to fuel cells, inverters,converters, reformers, and storage systems to improve reliability andavailability, without any major increase in capital cost overnon-modular designs, and to improve maintenance effectiveness and cost.Due to the scalability of the fuel cell, inverter, reformer, and storagecomponents, where scalability is exemplified when costs areapproximately proportioned to size, the modularizing and adaptivecontrol networking in this invention reduces capital cost burden. Thisis unlike other generation systems such as utility turbogenerators thathave capital cost economies of scale. Furthermore, the rapid startup ofproton exchange membrane (PEM) fuel cells may be relied upon to allownearly total shutdown when there is no load, and provides a significantadvantage over the startup and no-load costs experienced by othergeneration system alternatives.

Referring first to FIG. 1, a supply system 10 according to the presentinvention provides an output 11. Preferably the output 11 is a 120/240vac output to match a house wiring standard, although the output mayalso include dc as well as ac currents at selected voltages to operateparticular loads, for example, computer system boards, as required forthe stand alone facility to be powered by the system 10. A control 15fully integrates all elements of the fuel processing, electricalgeneration and power delivery to a plurality of loads coupled on theoutput 11. The control 15 includes a switch bank 14 having a pluralityof feeds 16 switchable to a plurality of loads to maximize continuity ofservice. Preferably, other components in addition to the feeds 16 arebanked to increase reliability and reduce costs, and subsystems areconfigured in at least (n+1) configurations, where (n) is the number ofnecessary subsystems to meet the specified operating capacity,availability, reliability and durability desires of the customer and(n+1) provides a spare subsystem.

In the preferred embodiment, the control 15 includes a controller 20having a clock generating an integrated time standard to avoid relianceupon a utility grid 60-cycle time reference that a grid provides at eachuser location. The elimination of a utility interface connection willremove the need for interface components with their attendant failuremodes and also isolate the variable fuel cell power supply from damagingtransients, harmonics, and lightning-induced damage originating on theutility grid. The controller 20 may be an integrated master controlmodule 22 or a network combining distributed processors, for sensing andcontrolling localized functions, with a link, for example, a commonprocessor, that orders the priority and response hierarchy to thedistributed processing. Nevertheless, the illustrated embodiments show acontroller 20, preferably comprising redundant controllers 22, to linkthe numerous sensors, actuators and distributed controllers that may beintegrated with such devices to provide a plurality of systemconfigurations, for operating the power supply system as a stand aloneunit, although the controller 20 is not so limited. The energy storagesystem 36 may be made of a plurality of storage cells, each of which maybe a battery, ultracapacitor, or preferably, a regenerative fuel cell.The storage system 36 buffers out load-induced transients and surges ordeficits from the fuel cell unit 30 and fuel-flow system 28 allowing thecontrol system 15 to gradually change fuel flows and decrease reformedfuel contamination and fuel cell stresses. The output 11 can provideboth ac and dc electrical current output through outputs from convertersystem 40, which may include a plurality of converters 42 includingdc-ac as well as dc-dc voltage converters, to eliminate the need fordedicated but inefficient ac-dc power supplies in appliances and inother electronic equipment loads to be operated by the system.

As a result, the present invention provides variable redundancy,banking, adaptive controls, modularity, and scalability features in thevariable fuel cell power supply including combinations needed for afree-standing electrical power generator with components of varyingdesign and production maturity to be reliable. The variable fuel cellpower supply design allows the tuning and optimization of the product tomeet customer requirements of reliability, durability, andmaintainability despite the constraints of supplied components andvariability in the operation of the components. The variable fuel cellpower system can be retrofitted and resized in the field, preferablyusing predetermined compatible plug-in modules and non-interruptingservice procedures by installing expansion modules in spare positionsand by hot-swapping failed modules to restore (n+1) spare operation.Sensors to detect this activity or control panel input willappropriately adjust the control algorithms.

Referring again to FIG. 1, a variable fuel cell power supply system 10is shown in a particular embodiment including at least two powerconversion units 12 coupled to the switch bank 14. A load bank 16,providing at least two ac power outlets 18, up to N number of outlets 18wherein N=the number of necessary outlets 18 for a particularapplication. In FIG. 1, the output 11 includes a plurality of loadcircuits 56 for a building or other facility. In this and the otherembodiments, each output subsystem 12, 14, and 16 includes appropriatesensor inputs for subsystem components, for example, for fuelparameters, electrical parameters, temperature, water and gas managementfor start-up, synchronization, and dynamic operation.

Such sensors may be integrated with distributed processor controllers,for example, as shown in FIG. 1, at any of 27, 28, 18, 62, 64, 66, 70,72, and 56, particularly where monitoring is necessary or the like forproper operation or compliance with regulations applicable to electricalpower supply, building codes, or the like. For example, feedback fromfuel cell controller 62 may provide one or more signals as a response tomonitoring a plurality of conditions, such as, fuel flow rate,temperature, impurity detection, leak detection, fuel volume and othersat stations such as fuel systems 28, air supply systems 27, fuel cellstacks 32, and reformers 29. Similarly, regulatory codes and standardsfor fusing, venting, and the like that are incorporated in each of thesubsystems shown, such as power conversion units 12 including sensor 64at storage units 36 and converter controller 66 at 40, switch bank 14,feeds to load bank 16, utility throwover switch controller 70, loadthrowover switch bank controller 72 and load sensors at 56.

Preferably, at least two sources of control, such as two control modules22 form a variable fuel cell power supply system controller 20 forintegrating the selected operations between power conversion units 12 inbank 13, the switch bank 14 and the load bank 16. Both manual controland automated control may be represented by the controller 20 althoughin the preferred embodiment, a single computer processor 22, preferablybacked up by a redundant processor 22, may manage the sensing, thresholdcomparison, and switch manipulation as will be described in greaterdetail below.

The loads and feeds 16 are monitored to allow automatic oralternatively, manual control, at each switch 24 in the bank 14 to matchcustomer selected load priorities and to prevent overloads underabnormal operating conditions. Likewise, monitoring of load circuits andfeeds will support predictive and preventive maintenance and recordperformance parameters so that reliability and durability are improvedover previously known residential power supply systems.

As shown in FIG. 1, each power conversion unit 12 includes a fuel cell30 including one or more fuel cell stacks 32, preferably made up ofproton exchange membrane (PEM) cells. Alternatively, solid oxide, moltencarbonate, phosphoric acid, alkaline, zinc oxide, or other fuel cellchemistries may be employed in a fuel cell 30. In the preferredembodiment, each fuel cell 30 is fed hydrogen-rich reformate from commonor modular reformers 29, preferably multi-fuel micro-channel reformers.However, hydrogen may also be provided from banked reformers, tanks,metal hydride or other sources alternative to the reformer 29 at source28. In addition, the fuel cell 30 is fed ambient air oxygen for the fuelcell's oxidant from the source 27, preferably ambient air fed through afilter 31. The independent fuel cells are also coupled to separatemodular dc energy storage devices 36, preferably a plurality ofregenerative fuel cells 38, or other storage devices as previouslydescribed. The plurality of storage devices 38 provide transient energybuffering and dynamic load following to dampen or smooth variableproduction at the fuel cells, and thereby dampen fuel flow transients atthe reformer to mitigate reformate contamination.

Each power conversion unit 12 contains at least one independent modularpower converter subsystem 40 to provide the desired voltage, current andwave form as a supply output 41. Preferably, converter subsystem 40comprises a plurality of independent modular converters 42, including dcto ac and dc to dc converters providing both ac and dc outputs.Preferably, a bank formed by a plurality of converters 42 provide adesired voltage current and waveform in a redundant manner so that aplurality of switching devices can connect the converter 40 to aplurality of ac or dc load circuits.

As also shown in FIG. 1, a double bus neutral (three wire) loaddistribution service panel will connect directly to a standard 120/240volt National Electric Code wiring standard building wiring system. Twooptional switches 44 operate in tandem, to allow manual throw over, orautomatic throw over, through switching controls 70, to an outsidecommercial feed or other electrical supply 57. Switches 24 in the bank14 provide a manual or automatic throw over of load circuits 56,preferably through automatic switching controls 72, to connect the loadcircuits 56 to the independent converter 40 outputs 41 connected throughthe load bus 50 or the load bus 52.

In a preferred embodiment, dual application specific microcomputers 22,which may be configured with power line carrier local communication 58,and remote digital or analog communications 60, are provided to controlthe steady state, transient and dynamic energy balance of the system.Preferably, controller 20 also includes a synchronizer, for example,clock processor, to synchronize the output of said power conversionunits. A preferred processor obtains a signal in compliance with theU.S. Naval Observatory clock time, to regulate the frequency of theoutput from dc-ac converter 40. An on-line monitoring function ofcontroller 20 can record performance parameters, and the controller 20is programmed with the logic that prioritizes the load circuits 56 whichmay receive power when limited capacity has been detected. Controller 20also activates required switching of the switches 44 and 24. Inaddition, controller 20 may predict or indicate preventative andreactive maintenance requirements, and may generate and deliver reportsboth locally and remotely that a threshold value of some performanceparameter has been attained at one of the sensors at 62, 64 and 66 forthe fuel cell unit 30, the storage unit 36, and the converter unit 40,respectively, that provide useful operating/performance information tothe controller 20.

Similarly, distributed controllers may be responsive to the processinglogic that is accomplished in the controller 20, such as a controller 70actuating terminal closure of switches 44 for load busses 50 and 52,respectively, and generating control signals for components of thevariable fuel cell power supply system 10, or selecting interaction withthe utility feed 57, as permitted by a master controller 22. Thecontrollers 72 may respond to or generate control signals for actuatingeach of the switches 24 in the bank 14, which similarly may beterminally engaged with load bus 50, or load bus 52 to provideselectable connections to the load bank 16. Preferably, the switches 24and 44 use break-before-make technology to avoid paralleling the supplysystem 10 and connector supply 57, to automatically throw over the loadbusses 50 and 52 to the connector supply 57, or to throw over loads fromone load bus to the other (50 to 52 or 52 to 50) without paralleling thesystem 10 with the connector supply 57 or paralleling load bus 50 withload bus 52.

Although the preferred embodiment has been disclosed as having automatedswitching control 70 and 72 in response to monitoring of performanceparameters and various components, it is to be understood that manualswitches may substantially reduce the cost of providing the samecapabilities without departing from the present invention and thereliability and robustness of the system due to the redundantcapabilities of each portion of the system.

Referring now to FIG. 2, a system 100 for banking power conversion units12 is shown and is particularly well adapted for modular application ofmicro-channel reformers where a separate reformer can be provided foreach unit 12 and a header fuel supply connected to the reformers. Eachpower conversion unit 12 in each bank 13 is equipped with a cutoffswitch 53 on its output that connects to a bus 50 or 52 for supplycircuit 102. The output 41 from each of the conversion units 12 normallyfeeds bus 50 or 52 of the common three wire 120/240 volt ac residentialwiring system. Upon the failure of a power conversion unit 12,controller 20 will automatically disconnect the unit 12 from the commonthree wire system at cutoff switch 53 and replace it with the spare unit67 by operating switch 55. The number of modules 12 in banks 13corresponds at least to the total required capacity of a load bus 50 and52, plus at least one spare 67. Alternately, at least one of theconversion units 12 may be redundantly backed up by a spare powerconversion unit 12 in each bank 13, as depicted in FIG. 2 by phantomline.

Referring now to FIG. 3, a system 110 is shown in which power conversionunit 12 consists of a bank of fuel cell stacks 32 that are coupledthrough the manifold 112 to a common source of oxygen 27, preferablyfrom filtered 31 ambient air, and a common source of purified hydrogen,preferably from a reformer 29, through the manifold 114. In addition, aplurality of dc storage units 38 are coupled in parallel for controlledswitching. Such switching may be in response to operation of thecontroller 20 or manual switching as previously discussed in order tobalance the flow of energy despite changes in the power supply deliveredfrom the power conversion units 12 in the bank 13, as well as to balancechanges in the loads being applied at output 56. A bank 116 of dc to acconverters 42 is similarly controlled in response to the controller 20to maintain and regulate sufficient current despite the changes in thenumber of loads applied to the output 41, and changes in the bufferingstorage facility 36 and changes in the operating parameters occurring ineach of the fuel cell stacks 32 in the fuel cell unit 30.

The output 41 may optionally be selectively engaged by utility interface45 and permits the power that remains unused in the building load 56 tobe fed into the commercial grid 57. As with the previously discussedmodules for power conversion, the interface 45 may be selectivelyoperated automatically in response to parameters monitored by thecontroller 20, although it is to be understood that it may be manuallyoperated without departing from the present invention. Preferably, thecontroller 20 includes communication capacity so that remote indicationsof the operating conditions occurring in numerous stacks, modules, fuelsupply and load circuits of the system may be delivered for indicatingor analyzing, for example, comparison with a threshold value for theoperating parameter, in order to determine whether a fault or overloadcondition exists. Moreover, the indicator may be provided locally asdesignated at 124 in FIG. 3, or it may be communicated remotely as shownat 126 in FIG. 3, or both.

A simplified conceptual example of improved VFCPS 10 performance isdepicted in FIG. 4 where the transient effect of motor starting is shownto be damped out and the energy balance in the system is maintainedthrough the natural discharge response of the storage unit 36 and theadaptive controller 20 of the fuel cell unit 30 output to supply the newload level and recharge the storage unit 36. A beginning VFCPS steadystate load P₁ is shown from T₀ to T₁ when the motor is started causingthe inrush rise to P₄. The motor starts up and the inrush subsidesresulting in steady state operation at T₂ with a VFCPS load of P₂.Energy area A1 depicts the natural response of the storage unit 36supplemented by the gradual adaptive controller 20 of the fuel cell 30.This combination damps the severe electrical transient and fuel supplytransient that would otherwise occur in the fuel cell system 28,29,30.Minimizing the fuel flow transient reduces the disturbance to thereformer 29 and consequently mitigates contamination, particularlycarbon monoxide, in the fuel supply 28 reformat to the fuel cell 30.Energy area A2 depicts the adaptive controller 20 raising the fuel cell30 output to P₃ to recharge the storage unit. The controller 20 thenreduces fuel cell 30 output to P₂ over time until the storage unit 36recharges and final steady state VFCPS operation resumes at time T₄.Thus, a combination of the natural energy response of subsystems and thesupplemental energy control of subsystems are utilized to maintain thetransient and dynamic energy balance of the VFCPS.

Referring again to FIGS. 1 and 4, the controller 20 may apply one ormore fuel cell stacks 32 using control 62 to gradually load them betweentimes T₁ and T₃. Likewise, any addition or removal of load by thecustomer (L₁ . . . L_(n)), or operation of a load throw-over switch 24or 44, can have similar transient/dynamic effects, responses, andresults. The controller 20 will also use controls 66, 64, 62, 70, 72,with load monitoring sensors to match subsystem supply capability andfull system supply capability to the corresponding connected loads. Thecontroller 20 and sensors will also monitor, detect, and isolatedefective subsystems 32, 38, 42 when necessary. The controller 20 with asubsystem including a plurality of computers, controls and sensors willbe configured as a redundant fault-tolerant system as is common in highreliability data processing applications. A goal of the controller 20,as depicted in FIG. 4, is to transform electrical transients/dynamicsand fuel flow transients/dynamics into gradual changes and steady stateoperation at the fuel cell 30 and at the reformer 29, respectively.Another goal of the controller 20 is to manage the activation anddeactivation of individual fuel cell and/or storage modules so that theoverall life expectancy of the system may be increased throughequalizing accumulated energy output for similar service life cells andapportioning accumulated energy output for dissimilar service life cellsof different service lives or different fuel cell chemistries. As aresult, the system is operational in a plurality of configurations so asto provide at least a useful supply of electrical power in afault-tolerant mode. Similar responses may be provided by the otherembodiments of the system, for example, load switching configurations,without departing from the scope and spirit of the present invention.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

The invention claimed is:
 1. A variable fuel cell power supply systemcomprising: at least one power conversion unit, wherein each said powerconversion unit comprises, a first unit including at least one oxygensource, at least one hydrogen source, and a plurality of fuel cellscoupled to said oxygen and hydrogen sources, a second unit including atleast one energy storage device coupled to said fuel cells, a third unitincluding at least one voltage converter device coupled to said fuelcells, and said energy storage device, wherein each of said fuel cells,said energy storage device, and said voltage converter device areindividually switchable; at least one load bus, each load bus connectedto said power conversion unit at said third unit of voltage converterdevice; at least one load coupler, each load coupler selectivelyconnecting said at least one load bus to a load line; and at least oneprocess controller responsive to a plurality of operating parametersfrom said power conversion unit and said load coupler, wherein saidprocess controller is configured to selectively engage at least one ofsaid plurality of fuel cells, which dynamically adjusts the totalgenerating capacity of all of said fuel cells, to substantially meet thechanging power needs of the load.
 2. The invention as described in claim1 wherein said voltage converter device comprises at least one dc-dcconverter.
 3. The invention as described in claim 1 wherein said voltageconverter device comprises at least one ac-dc converter.
 4. Theinvention as defined in claim 1, wherein said process controller furtherincludes a switch control comprising: at least one sensor providingoutput in response to one of said operating parameters; a detectordetermining when said output attains a threshold value; an indicatordemonstrating the attainment of said threshold value; and a manipulatorto change the state of at least one of said fuel cells.
 5. The inventionas defined in claim 1, wherein said process controller further includesa switch control comprising: at least one sensor providing output inresponse to one of said operating parameters; a detector determiningwhen said output attains a threshold value; and an automatic switchcontroller responsive to said threshold value to change the state of atleast one of said fuel cells automatically.
 6. The invention as definedin claim 1, wherein said operating parameters further include at leastone load coupler parameter, and wherein said process controller furtherincludes a monitoring processor comprising: at least one sensorproviding output in response to one of said operating parameters; amonitor to record and communicate at least one operating parameter; andat least one communications channel to communicate said operatingparameter.
 7. The invention as defined in claim 1, wherein said processcontroller further comprises a synchronizer for synchronizing outputs ofsaid power conversion unit.
 8. The invention as defined in claim 1,wherein the process controller comprises a plurality of controllers thatmonitor said plurality of controllers and said operating parameters forselectively switching to one of said plurality of controllers for masterstatus.